Question 447118
{{{ d = 7200 - 800t }}}
The graph is:
{{{ graph( 400, 400, -5, 10, -1000, 10000, 7200 - 800x) }}} 
The plane begins  its flight at Toronto at {{{t = 0}}}
Putting this into the equation:
{{{ d = 7200 - 800*0 }}}
It has 7200 km to fly to reach Rome
If it flies 4000 km, It will have {{{ 7200 - 4000 = 3200 }}}
km to go to reach Rome
Putting this into equation:
{{{ 3200 = 7200 - 800t }}}
{{{ -800t = -4000 }}}
{{{ t = 5 }}}
After traveling for 5 hours it will be 3200 km from Rome
and it will have covered 4000 km
This agrees with the graph
Notice that when {{{d = 0}}}, it has reached Rome
{{{ 0 = 7200 - 800t }}}
{{{ 800t = 7200 }}}
{{{ t = 9 }}}
This says the whole trip takes 9 hrs